Since this person is *trying* to do right by you, it's probably best to approach it by first acknowledging their attempts to be helpful before asking them to stop.  I'd recommend privately saying something like:

>"I notice you've been sticking up for me when ______ (instance/example), and I appreciate the intent behind you doing that for me. Can I ask you to do me a favor? I'm not bothered by _______ so I'd prefer to be the one to say something if I was."

The key to not making this relationship turn ill (especially since this person seems to be a workplace acquaintance) is to pat this person on the back for attempting to be your ally *before* telling them that you'd prefer to fight your own battles.  While you may not actually be thankful for their attempts, it'll make the rejection a bit easier for them to handle (because at least you're saying you appreciate their effort).

Additionally, asking them to comply to your wishes via leading with "Can I ask you to do me a favor?" also let's them continue to take action in being your ally (but by letting you handle it yourself).  They clearly *want* to help, so asking them to hold their tongue for you may be a great way to redirect their undesired behavior to a desired one (in your case, them keeping quiet).

**tl;dr: thanks, but no thanks**

On the side of approaching those who were "scolded" by this person, they'll probably warm back up to joking around after a while of not having this third party run interference (especially since you seem to like to joke about it yourself, and that's sure to lighten them back up to it).  

I'd avoid telling them that the acquaintance was out of line because, again, he *was* trying to help... Plus, it could reflect poorly on you if it got back to him that you were actually really put off by his actions, and end up causing tension on your relationship.